78 CARBONIFEROUS LAMELLIBRANCHIATA. 



that the shell is a little crushed. The lines of growth are separated at intervals 

 by more deeply cut interruptions. The second specimen on the plaque has not 

 these distinctive characters, but I hesitate for the present to place M. Cordolianns 

 as a synonym of LWiodomus lingualis. 



LiTHODOMUS LiTHODOMOiDES, U. Ethevldge, juii., 1875. Plate I, figs. 6, 6 a, 6 h, 7, 8. 



MoDiOLA LITHODOMOIDES (pars), B. Etheridge, jun., 1875. G-eol. Mag., dec. 2, 



vol. ii, p. 211, pi. viii, fig. 2. 



Specific Characters. — Shell transversely elongate, fusiform, almost cylindrical 

 in front, flattened and expanded behind, very slightly oblique. The anterior end is 

 obsolete. The umbones project in front of the rest of the shell, and are only very 

 slightly marked oflP from the anterior portion of the shell. The anterior border 

 is obtusely rounded and very narrow. The ventral border is convex in front and 

 behind, concave in the middle third. In the middle and posterior thirds, near the 

 ventral border, the shell becomes bent on itself inwards at right angles, so as to 

 present a narrow inferior surface, the extreme edge of each valve being then 

 directed upwards, so that there is a narrow elongated trench, along the bottom of 

 which the valves come in contact. The byssal opening is in the front part of this 

 trench, as in Modiola now inhabiting the Firth of Forth. The posterior border 

 is lanceolate, very obliquely truncate above, and obtusely rounded below, joining 

 the ventral border with a very obtuse curve. The hinge-line is straight, equal in 

 length to about two-thirds of the extreme antero-posterior measurement of the 

 shell ; for the greater part of its extent depressed below the level of the valves, but 

 elevated above them where it joins the posterior border, into which it passes with 

 a very obtuse angle. The shell is convexly swollen in the anterior two-thirds, 

 especially above ; but below it is only slightly compressed into the ventral margin, 

 towards the middle of which the shell becomes broadly compressed, hollowed 

 more especially in the right valve. Behind, the shell is compressed, flattened, and 

 expanded. The umbones are anterior, elevated above the hinge edge, and remote. 

 From the upper edge of each a ridge proceeds directly backwards, between which 

 and the edge of the valve is an elongated concavity, which, commencing narrow, 

 becomes broader by encroaching on the valve, and then gradually approximates 

 to the central line to meet the edge of the valve, where it becomes the highest 

 part of the shell. 



The interior is unknown. 



Exterior. — The surface is covered by fine and close strice and lines of growth. 

 Here and there towards the ventral margin oue of the lines becomes accentuated 

 and raised a little above the level of the others. 



